A distributed denial-of-service (DDoS) attack is a malicious attempt to disrupt the normal traffic of a target server, service, or network.
A hallmark of DDoS attacks is the distributed nature of the malicious traffic, which usually comes from a botnet - a criminal-controlled network of compromised machines scattered around the world.
Over the years, cybercriminals have developed a number of technical approaches that tend to fall into three general types of DDoS attacks. Read more at this link.
https://pentestapp.io/how-to-test-ddos-protection/